Crystalline polypropylene, typically a homopolymer, is used extensively in various moldings because it exhibits desirable mechanical (e.g., rigidity) and chemical resistance properties. For applications that require impact resistance (e.g., automobile parts, appliance facia, packaging, etc.), a rubber, e.g., copolymer of propylene and ethylene and/or one or more α-olefins, is used, or a blend of crystalline polypropylene with one or more rubbers that exhibit good impact resistance, e.g., propylene/ethylene (P/E) copolymer, or ethylene-propylene (EP) and/or ethylene-propylene-diene (EPDM) rubber. Crystalline polypropylene has an isotactic structure, and it is readily produced using a Ziegler-Natta (Z-N) or a metallocene catalyst, or a constrained geometry catalyst (CGC). For purposes of this disclosure, P/E copolymers comprise 50 weight percent or more propylene while EP copolymers comprise 51 weight percent or more ethylene. As here used, “comprise . . . propylene”, “comprise . . . ethylene” and similar terms mean that the polymer comprises units derived from propylene, ethylene or the like as opposed to the compounds themselves.
Polypropylene impact compositions typically comprise (i) one or ore matrix polymers, e.g., a crystalline polypropylene homo- or copolymer, and (ii) one or more impact modifiers, typically a rubber. The matrix provides the stiffness and optical properties, and the impact modifier provides the toughness. The addition of an impact modifier generally causes a reduction in stiffness and optics of the total blend compared to the stiffness and optics of the matrix by itself. This reduction in optics can be minimized by carefully designing the solubility of the impact modifier with regard to the matrix.
The solubility of a propylene-ethylene impact modifier in the matrix is determined by composition and molecular weight. To achieve sufficient toughness, a minimal level of ethylene is required in the propylene-ethylene impact modifier, which then leaves the molecular weight as the parameter to influence solubility. Decreasing molecular weight of the impact modifier increases the solubility of the impact modifier in the matrix. However, impact modifiers with low molecular weight may migrate to the surface of fabricated parts and show blooming which can reduce the optic performance significantly.
The addition of the impact modifier can be in-situ (e.g., reactors in series) or off-line via compounding (e.g., physically blending the matrix and impact modifying resins). Impact modifiers can be more accurately designed via single site catalysis (e.g., by a metallocene) than via Ziegler-Natta catalysis. Additionally, impact modifiers prepared via single site catalysis have a narrow molecular weight distribution (MWD), and thus have less low molecular weight extractables than impact modifiers prepared via Ziegler-Natta catalysis.